When mixing at least two fluids, the objective is to achieve a uniform distribution as rapidly as possible. It is advantageous to use the static mixers described by W. Ehrfeld, V. Hessel, H. Löwe in Microreactors, New Technology for Modern Chemistry, Wiley-VCH 2000, p. 41–85. Known static mixers achieve mixing times for liquids between several milliseconds and 1 second by generating alternate adjacent fluid layers of micrometer range thickness. The higher diffusion constants for gases provide even more rapid mixing. In contrast to dynamic mixers, where turbulent flow conditions prevail, the predetermined geometry of static mixers allows precise fixing of the fluid layer widths and diffusion paths. As a result, a very close distribution of mixing times is achieved. This allows numerous possibilities for optimizing chemical reactions with regard to selectivity, yield, and even safety.
A further advantage of static mixers is a reduction in component size, allowing greater ease of integration with adjoining equipment, such as heat exchangers and reactors. Process optimization may also be enhanced due to forced interactions between two or more components within a confined space. Static mixers apply to forming not only liquid/liquid and gas/gas mixtures, but also liquid/liquid emulsions and liquid/gas dispersions. Static mixers have also found use in multiphase and phase-transfer reactions.
A static mixer operating using the principle of multilamination or fluid layering has, in one plane, a structure of intermingled channels of a width of about 25–40 microns (i.d., pp. 64–73). The channels divide two fluids to be mixed into a multiplicity of separate fluid streams, arranged to flow parallel and alternately in opposite directions. Adjacent fluid streams are removed vertically upward out of a horizontal plane and through a slot and are brought into contact with one another. Using structuring methods suitable for mass production, however, the channel geometries and therefore the fluid layer widths can be reduced to the submicron range to only a limited extent.
A further reduction in the size of fluid layers using the multilamination principle is achieved by so-called geometric focusing. A static mixer using this principle for reacting hazardous substances is described by T. M. Floyd et al. in Microreaction technology: industrial prospects; proceedings of the Third International Conference on Microreaction Technology/IMRET3, W. Ehrfeld, Springer 2000, pp. 171–179. Alternately adjacent channels for the two fluids to be mixed open outward in a semicircle, radially from the outside, into a chamber extending into a funnel shape and merging into a narrow, elongate channel. The layered fluid stream is combined in the chamber and then transferred to the narrow channel, so that the individual fluid layer width is reduced. Under these laminar flow conditions, mixing is purely diffusional. Therefore, mixing times in the millisecond range are achieved by reducing the fluid layer width to the submicron range. A drawback with this configuration is that the narrow channel must be sufficiently long to achieve full, intimate mixing. This requires a large structure and promotes relatively high pressure loss.
In contrast to these disclosures, the present invention provides a solution to the well-known problem of mixing at least two fluids rapidly and uniformly, while at the same time maintaining low pressure drop characteristics and an economical design. The efficient mixing provided is especially useful in combination with chemical reactions where extremely good dispersion of reactants can overcome diffusion limitations and/or even reduce hazards where an explosive mixture of feed components is involved. In terms of the latter benefit, the present invention can be integrated with many types of oxidation reactions or those involving selective combustion to provide internal heating. Oxidation reactions to which the present invention applies include, for example, the direct oxidation of ethylene to ethylene oxide, as described in U.S. Pat. No. 4,212,772. Another reaction of particular interest is in the manufacture of hydrogen peroxide from hydrogen and oxygen, described in U.S. Pat. No. 4,832,938.
Currently the most widely practiced industrial scale production method of hydrogen peroxide is an autooxidation process employing alkylanthraquinone as the working material. This process comprises dissolving alkylanthraquinone in an organic working solution to perform reduction, oxidation, separation by aqueous extraction, refining, and concentration operations. Overall, the use of a solvent phase adds complexity and requires high installation and operating costs.
Considerably more simple and economical than the alkylanthraquinone route is the direct synthesis of hydrogen peroxide from gaseous hydrogen and oxygen feed streams. However, this approach carries the serious risk of explosion of the gaseous mixture of feed components in stiochiometric quantities. It is well known that oxygen-hydrogen gaseous mixtures have one of the greatest potentials for explosion. That is, explosive concentrations of hydrogen in an oxygen-hydrogen gaseous mixture at normal temperature and pressure are from 4.7% to 93.9% by volume. Thus the range is extremely broad. It is also known that dilution of the gaseous mixture with an inert gas like nitrogen scarcely changes the lower limit concentrations of the two gases. Within normal ranges of pressure variation (1–200 atmospheres) and temperature variation (0–100° C.) the explosive range is known to undergo little change.
In contrast to the prior art, the present invention uses highly effective mixing of a potentially explosive mixture of reactants to overcome the inherent safety considerations. Essentially, the rapid and complete mixing allows the reactants to be chemically transformed without any significant amounts of unreacted components being present in explosive concentrations for any significant length of time. The present invention is therefore suitable for a number of oxidative and combustive reactions. Prior to reaction, the feed components are mixed in a manner utilizing a vortex or mixing chamber that promotes complete mixing without significant pressure loss. Although the invention may be used in a wide variety of applications, the invention is particularly suited for small-scale or micromixing operations that are coupled with reaction.
In the specific case where the present combined mixing/reaction process is used in the preparation of hydrogen peroxide through the direct reaction of hydrogen and oxygen, a considerable cost savings is realized over the above mentioned alkylanthraquinone route. A cheaper method of hydrogen peroxide production also favorably impacts the economics of downstream uses, such as in the further reaction of hydrogen peroxide with propylene to form propylene oxide.